Although frogs may first come to mind when we think of ectothermic
vertebrates with a proclivity for jumping, it should be noted that many
arboreal lizards are no strangers to this mode of locomotion. What makes a
lizard a good jumper? What trade-offs might good jumping impose? In a recent
paper, Esteban Toro, Anthony Herrel and Duncan Irschick explore the
relationships among morphology, ecology and jumping in Caribbean
Anolis lizards to answer such questions and to address more generally
the evolution of ballistic locomotory behavior within this clade.

As a genus, Anolis is huge, encompassing over three hundred
species of lizard, many of which live among the islands of the Caribbean. What
has made these Caribbean animals so attractive to evolutionary biologists (in
addition to necessitating a tropical field-site visit) is that in their
radiation across the islands, convergent evolution has molded many species
into six identifiable `ecomorphs': species that share common features related
to their habitat use and morphology, but have largely evolved these ecological
and anatomical similarities independently. Presumably, some of the anatomical
traits that are characteristic of a particular ecomorph represent adaptations
to a specific habitat, or way of life. A number of scientists have taken
advantage of this system to begin to study the process of adaptation by
examining the links between performance, structure and ecology. In this most
recent example, Toro and colleagues explore the mechanics of jumping across
Anolis ecomorphs to understand how jumping function relates to limb
morphology.

Up to ten individuals from each of 12 species comprising all six ecomorphs
were studied while jumping off a custom-made force plate. Performance
variables, such as take-off angle, distance, peak velocity, acceleration and
power, were measured from each individual's best jump. Morphometric variables
were also determined for each animal. The results showed that the lizards, on
average, used significantly lower take-off angles (∼36°,
range=31–40°) than predicted for optimal jump distance
(39–42°), and the authors did not find a strong correlation between
the animals' take-off angles and their preferred habitat. As the authors point
out, jump distance may not be the only relevant variable for a jumping lizard.
Minimizing flight time might also be important and, indeed, at these slightly
lower take-off angles, lizards forsake only a small loss in distance (∼1%)
for a more substantial reduction in air-time (∼7%). Perhaps the use of
lower-than-optimal take-off angles represents a balance between maximizing
jump distance while simultaneously minimizing flight time.

Among the morphological variables, only hind-limb length was a positive
predictor of take-off velocity (which, in turn, is directly related to jump
distance). This makes intuitive sense as longer limbs increase the time and
distance over which the body can be accelerated before take-off. Previous work
on Caribbean anoles has shown that relative limb length can impact running
ability on narrow surfaces, such as thin branches. Specifically, long-limbed
animals perform poorly on narrow surfaces. Thus, evolving long limbs may
improve jumping distance, but probably at the cost of reduced running agility
along small tree branches. Intriguingly, some shorter-limbed species may have
solved this trade-off by increasing hind-limb extensor-muscle mass, allowing
enhanced acceleration despite short limbs.

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